New Strain of MRSA Found in Milk

June 2, 2011 -- Researchers have discovered a new strain of antibiotic-resistant superbug bacteria in milk. This previously undetectable strain has also caused human infections.

The bacterium, a strain of methicillin-resistant Staphylococcus aureus (MRSA), appears to be relatively rare. It turns up in about 1% of MRSA cultured from humans in the U.K.

Researchers say it poses little threat to people who drink milk or eat dairy products like cheese, since pasteurization and digestion kill bacteria, including MRSA.

Any danger to people, researchers say, would likely come from contact with cows that carry the strain.

Whether or not the new strain may be present in cattle or milk in the U.S. is an open question.

"The main worry would be that these cows represent a pool of the bacteria and these bacteria end up colonizing people that work or live on farms and they take it out to the wider community," says study researcher Mark A. Holmes, VetMB, a senior lecturer in the department of veterinary science at the University of Cambridge, England.

Of greater concern, they say, is the fact that this new MRSA strain carries a gene that allows it to elude detection by current "gold standard" polymerase chain reaction (PCR) tests that are favored by hospitals and labs for their speed.

"If you end up with a serious infection from this bacteria and your sample goes to a laboratory to be tested and the only means of testing they do is the PCR testing, you could be falsely negatively diagnosed, be given methicillin-like drugs to treat it, and they would be ineffective," says Holmes.

Some Tests May Miss MRSA

The study researchers identified the new strain because they tested for bacteria using two methods: one in which scientists swab bacteria on a gel food called agar that's dotted with antibiotic-impregnated discs to see if the drugs can kill the bacteria; and the second, PCR testing, which involves looking for a gene called mecA that makes the pathogen resistant to a host of antibiotics, including methicillin.

The agar plate test showed that new strain was antibiotic-resistant, but curiously, the PCR test did not find the mecA gene.

It wasn't until scientists sequenced the entire genome of the bacterium that they discovered the reason for the discrepancy.

The new strain had a copycat gene, called a homologue, which still conferred antibiotic resistance but couldn't be detected on PCR because it doesn't exactly match the reference sample.

Researchers say it's a problem akin to trying to search a document on a computer but misspelling the search term.

"If you're trying to find the word Staphylococcus in a great long document," Holmes says, "and you type in Staphylococcus in the search, it will find it if you spell it right, but if you actually get one letter wrong, it's not going to find it."

Increasingly, hospitals and labs have moved away from using the agar plate sensitivity test because it is slower, requiring up to three days for results. Rapid PCR testing, on the other hand, can take just 30 minutes.

With greater reliance on PCR, experts say, the new finding of homologue genes that can fly under its radar suggests that some cases of MRSA are probably being missed.

Indeed, when researchers then tested samples of bacteria cultured from humans that were kept by reference labs around Europe, they found the new strain in the U.K. and in Denmark.

Many of those samples were unexplained cases of antibiotic resistance, some from patients with serious infections and others that were found through routine screening tests, which had been kept in freezers for further analysis.

The new strain helped to solve only about half of the unexplained cases, experts said.

"There's more out there that we don't know yet," says Professor Ruth N. Zadoks, DVM, PhD, Moredun Chair of Veterinary Epidemiology at the University of Edinburgh, Scotland.

Zadoks called the new study "fantastic detective work" that will have "considerable public health impact."

The study is published in The Lancet.

Is MRSA in U.S. Milk?

Zadoks has tested U.S. milk samples for the presence of MRSA but was not involved in the current research.

In a 2009 paper published in the Journal of Dairy Science, she reported finding no MRSA after testing milk from 542 U.S. farming operations.

Nine of her samples, however, did test positive for the mecA gene, indicating antibiotic resistance.

The U.K. researchers say that because they did not test U.S. milk samples or bacterial cultures, they can't say whether or not the new strain may have reached this country.

"It's possible," Holmes says. "I have no doubt at all that over the next few months they'll be a flurry of activity of people just checking to see if they can pick it up on any of their farms."